MAINTAINERS: remove myself as nouveau maintainer

I have resigned, and will no longer be taking as active a role in
nouveau development.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230918222225.8629-1-skeggsb@gmail.com